By Damian Paletta and William Mauldin 

WASHINGTON--The Obama administration is prepared to expand a new set of economic sanctions against Russia if the country doesn't take steps to end Ukraine's conflict with pro-Russia separatist fighters, a senior administration official said.

U.S. officials, who are looking to raise pressure on President Vladimir Putin, on Wednesday imposed a new type of sanctions which limit the financing activities of some of Russia's biggest companies, including state-controlled oil giant OAO Rosneft. Since then, the downing of Malaysia Airlines flight 17 has broadened support in the U.S. and Europe for using economic levers against Russia, even if the measures also affect Western companies.

Under the new sanctions program, the Treasury Department could expand the number of institutions affected and expand the limitations placed on the Russian firms, the senior administration official said. The current rules prevent people or firms with ties to the U.S. from providing medium- and long-term credit to Rosneft, Russia's second-biggest natural-gas producer and two major financial institutions. The Treasury also bars equity financing for some of the firms.

The official didn't elaborate on what a broader array of limitations might look like. The current sanctions don't prevent U.S. entities from doing business with the Russian firms or freeze their assets.

The current set of sanctions was designed so it could be gradually intensified over time if Russia doesn't work to seal the border and prevent the flow of weapons to the separatist fighters. White House officials have aimed to avoid any economic spillover in the U.S. from the punitive measures, the official said.

But Obama administration officials are prepared to take more extreme steps if necessary that could have a broader economic impact to force Russian cooperation, the official said.

On Tuesday, European Union ministers expanded their sanctions program against Russia, and the names of the new people and entities targeted are expected to be released later this week. Officials in Brussels are also developing a set of broader economic sanctions that could target the defense, energy By And or financial sectors.

Write to Damian Paletta at damian.paletta@wsj.com and William Mauldin at william.mauldin@wsj.com